Polyether, polycarbonate, polyester and poly(ester-carbonate) are important functional polymer materials that can be prepared from epoxides homopolymerization, copolymerization of epoxides with monomers such as CO2 or anhydride. Among these materials, polycarbonate, polyester and poly(ester-carbonate) are new kind of degradable polymer materials and their monomers such as CO2 and anhydrides are abundant and inexpensive. Therefore, these materials have very significant development promises. The key issue for synthesizing these (bio) degradable polymers is to obtain complete alternative copolymers.
Especially, for aliphatic polycarbonates (APCs), which are produced by the copolymerization of CO2 and epoxides, the key characteristics of their application are their biodegradability and oxygen barrier properties. These properties are mainly determined by whether the molecular structure of the copolymer is completely alternating. According to current technology, heterogeneous catalysts such as zinc glutarate catalyst and rare earth metal ternary catalyst system could catalyze the full alternating copolymerization of CO2 and epoxides. However, they exhibited very low productivity. Usually, the best catalyzing activity is about 60 g copolymer/g catalyst. Because of the low activity and a large amount of the required catalyst, it is difficult to separate the catalyst from the product, which causes high production cost. On the other hand, when homogeneous catalysts such as zinc bis (β-diiminates) catalyst, or Salen-Co catalyst are employed as catalysts for the CO2-epoxides copolymerization, complete alternating copolymer are achieved successfully with an activity of above 100 g polymer/g catalyst (up to about 900 g polymer/g catalyst). However, these homogeneous catalysts require expensive raw materials, long and strict synthetic route, rigorous reaction condition, as well as the difficulty in separating the catalysts from reactive products.
Traditional double metal cyanide (DMC) complex is an efficient catalyst for epoxides homopolymerization to produce polyether polyols, which presents the advantages of very low unsaturation degree (0.005˜0.008 mol/kg) and narrow molecular weight distribution (Mw/Mn<1.2), and thus is clearly better than those polyols made by using KOH catalyst. However, due to the DMC catalyst's high activity, which usually results in fast polymerization and strong exothermal effect in a short reaction time after the induction stage, such catalyst may lead to the generation of products with high molecular weight and process serious danger during polymerization. As a result, ensuing a smooth progression of the catalyzing process and eliminating high molecular weight end products are the keys to obtain high quality polyols.
In recent years, the high activity of the DMC catalysts also attracted researchers to use them in the reactions for the copolymerization of CO2 and epoxides to produce polycarbonate and the copolymerization of CO2 and anhydride to produce polyester. Very regrettably, there are various drawbacks in current technology using DMC to catalyze the copolymerization of epoxides and CO2. These drawbacks mainly include the resultant polymer containing a significant portion of polyether, low carbonate units, and a relatively high content of cyclic carbonate byproduct. This is because the generation of polyether and cyclic carbonate are thermodynamically favorable. The current published DMC catalysts catalyzes propylene oxide (PO)/CO2 copolymerization to obtain poly(ether-carbonate)s with a low molecular weight or a low glass transition temperature. The product's thermal characteristics, such as glass transition temperature (Tg), and the thermal decomposition temperature (Td) were also unsatisfied. Moreover, massive production of cyclic carbonate byproduct caused waste of propylene oxide monomers and complexity in post-reaction processing.
According to current technology, such as those methods reported in Chinese Patent Application No. 200680010849.0 and No. 200780027326.1, DMC catalysts were used for making poly(ether-carbonate). The structure of the polymerization products includes a significant amount of polyether. The polymerization products contain a significant amount of cyclic byproducts and the molecular weight is low. The DMC catalyst disclosed by No. 200680010849.0 showed a polymerization activity of about 0.8 kg polymer/g catalyst at the highest.
Moreover, U.S. Pat. No. 4,500,704 discloses that a Zn—Co double metal complex catalyst with 2-methoxyethanol as a complexing agent was used to catalyze PO—CO2 copolymerization under 700 psi polymerization pressure and at 35° C. for 48 h. The resultant polymer showed a high molecular weight of 23000. However, its glass transition temperature Tg was only 8° C., suggesting that the carbonate chain units content was rather low (complete alternating PO/CO2 copolymerization products with a high molecular weight MW exhibit a Tg>35° C.). In addition, the technologies of using double metal cyanide catalyst to catalyze CO2 copolymerization disclosed in U.S. Pat. No. 6,713,599, U.S. Pat. No. 6,762,278 and U.S. Pat. No. 4,826,053 have the same problems. That is, the products have low molecular weights; the weight percentage of polycarbonate is generally lower than 20 wt. %; a high cyclic carbonate content in the products as well as a low catalyst productivity.
From the perspective of the catalysis principle, the catalytic performance of the heterogeneous DMC catalyst is internally determined by the structure of the active site and externally influenced by the specific surface area of the catalyst. The current technology mainly improves DMC catalyst through alternation of the external complexing agents, addition of salt and a supporter. These methods are usually processed in water, thus usable organic external complexing agents are limited. Most oil-soluble organic solvents are difficult to form strong or weak coordination structure with active metals in the water phase, and in fact it is difficult to substantially modify the environment and the coordination structure of the active sites. Thus, the catalyst performance of the catalyst made from the current technology was not significantly improved for epoxides-CO2 copolymerization compared to traditional DMC catalysts. In conclusion, it is really an unsolved challenge for a DMC catalyst to catalyze CO2-epoxides copolymerization to achieve full alternating polycarbonate with a high molecular weight, high productivity, high selectivity and low cyclic carbonate byproducts.